A filter composed of glass having a deposited film thereon and a filter composed of phosphate glass containing metal ions are known as infrared absorption filters. However, these glass infrared absorption filters are heavy, easily broken and hard to be processed, for example, bent.
To overcome these defects, a plastic infrared absorption filter is now under study and many materials are proposed. For example, JP-A 2000-227515 (patent document 1) discloses an infrared absorption filter prepared by forming a coating layer made of a composition containing an infrared absorbing coloring matter dispersed in a binder resin on a base material, wherein the amount of the residual solvent in the laminate is 5.0 wt % or less. In this document, polyester-based resins, acrylic resins, polyamide-based resins, polyurethane-based resins, polyolefin-based resins and polycarbonate-based resins are enumerated as examples of the binder resin.
It is desired that the plastic infrared absorption filter should exhibit infrared absorptivity with a small film thickness. Therefore, the coloring matter contained in a thin film formed after the solvent is removed must be uniformly dispersed in the binder resin in a high concentration without being maldistributed, agglomerated and precipitated to the surface. Further, the coloring matter dispersed in the binder resin must retain durability for a long time, be free from a reaction with another coloring matter and not be deteriorated by the environment such as heat, light and moisture.
Since most coloring matters used as an infrared light absorber are very unstable to light, heat and water, the binder resin must serve as a protection layer for preventing the deterioration of the coloring matters. However, the above transparent resins which are well known as the binder resin are not satisfactory in terms of the above function, and various resins are proposed instead.
For example, JP-A 11-116826 (patent document 2) proposes use of a polyester resin containing a diol component having a specific structure in order to disperse a coloring matter stably in the resin. This document teaches that a polyester prepared by copolymerizing a diol having a fluorene skeleton may be used as the resin.
Japanese Patent No. 3308545 (patent document 3) discloses a multi-layer infrared absorption film which comprises an infrared absorption film containing an infrared absorbing coloring matter such as a phthalocyanine-based metal complex dispersed in a transparent polymer resin and a multi-layer infrared absorption film having at least one of an electromagnetic wave absorbing layer, an antireflective layer and an ultraviolet absorbing layer. In this document, copolyesters, polymethyl methacrylate, polystyrenes, amorphous polyolefins, polyisocyanates, polyacrylates and triacetyl celluloses are enumerated as examples of the polymer resin. A copolyester comprising 0.4 mol of dimethyl terephthalate, 0.88 mol of ethylene glycol and 0.28 mol of 9,9-bis(4-(2-hydroxyethoxy)phenyl)fluorene is used as the polymer resin (Example 1). However, it is desired to further improve the dispersibilities of these resins so that the infrared absorbing coloring matter can be dispersed uniformly in a high concentration. Also, there remains a problem with the durability of the obtained infrared absorption filter.
(Patent Document 1) JP-A 2000-227515
(Patent Document 2) JP-A 11-116826
(Patent Document 3) Japanese Patent No. 3308545